This application claims the benefit of Japanese Application Nos. 2000-093619, filed on Mar. 30, 2000, and 2001-027661, filed on Feb. 5, 2001, which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, in which exhaust emission purifying catalysts for purifying toxic substances in exhaust gases are disposed in an exhaust passage of the internal combustion engine.
2. Description of Related Art
Ordinarily, an exhaust system of an internal combustion engine is provided with a three-way catalyst that purifies carbon monoxide (hereinafter referred to as CO), hydrocarbon (hereinafter referred to as HC), and nitrogen oxide (hereinafter referred to as NOx) in exhaust gases as disclosed in Japanese Patent Publication No. 8-14029.
In an xe2x80x9cexhaust emission control devicexe2x80x9d disclosed in the above publication, an upstream catalyst and a downstream catalyst comprised of a three-way catalyst are disposed in an exhaust passage of the engine. The upstream catalyst oxidizes the CO and the HC in exhaust gases at an inflow side thereof, and oxidizes the CO and reduces the NOx at an outflow side thereof; whereas the downstream catalyst oxidizes the CO and reduces the NOx. This improves an oxidation-reduction capability of the catalysts and activates the exhaust gases.
The above xe2x80x9cexhaust emission control device,xe2x80x9d however, has a disadvantage that a catalytic reaction may degrade according to an operational status of the internal combustion engine, a catalyst temperature, a change due to passage of time, and the like. The degradation of the catalytic reaction results in the degradation of an exhaust emission purifying reaction. It is, therefore, impossible to achieve a stable exhaust emission control performance.
It is, therefore, an object of the present invention to provide an exhaust emission control device for an internal combustion engine, the device which properly regenerates degraded catalysts to improve an exhaust emission control performance.
To attain the above object, the present invention provides an exhaust emission purifying device for an internal combustion engine, the device comprising: exhaust emission purifying catalysts for purifying toxic substances in exhaust gases, the exhaust emission purifying catalysts being disposed in an exhaust passage of the internal combustion engine; and a control unit for temporarily turning the exhaust gases flowing into the exhaust emission purifying catalysts into oxide atmosphere if the exhaust gases flowing into the exhaust emission purifying catalysts are turned into reduced atmosphere and continue to be the reduced atmosphere for a predetermined reference period of time or longer.